1. Field of the Invention
The present invention pertains to a method and apparatus for preventing unwanted objects from entering portions of a wellbore. More particularly, the present invention pertains to the use of magnetic field(s) to catch unwanted objects within a wellbore. More particularly still, the present invention pertains to an apparatus for generating magnetic field(s) between a rig floor of a rig and a wellhead, such as in a marine riser assembly, in order to prevent unwanted ferrous objects from entering a wellbore below said wellhead.
2. Brief Description of the Prior Art
Objects can sometimes accidentally fall into the uppermost opening of a well from above including, without limitation, from the rig floor surrounding said opening. In other instances, objects can be purposely thrown or dropped into a well as an intentional act of sabotage. If such objects are not stopped before entering the subterranean portion of the wellbore, the objects can prevent downhole equipment from functioning properly and can often impede the drilling and/or completion process.
Relatively large objects can generally be retrieved from a wellbore using specially designed “fishing tools.” Such fishing tools are lowered into a wellbore and connect to a dropped object within the wellbore. Thereafter, the fishing tools and the connected object can both be safely retrieved from the wellbore. In many instances, the retrieval process for such large objects can be relatively simple because the size of the objects enables such objects to be grasped and lifted out of the wellbore. In oil and gas operations there is usually significant expense associated with having to use fishing tools.
By contrast, relatively small objects dropped in a wellbore—and particularly ferrous metal objects—can often cause the most disruption to downhole equipment and related operations. For example, during completion operations, small pieces of metal present in a wellbore can prevent packers and other completion tools from sealing against a casing wall. During open hole drilling operations, such small metal objects can destroy very expensive downhole equipment such as Polycrystalline Diamond Compact (PDC) bits.
Such small objects can also be very difficult to retrieve from a wellbore, as they are often too small to be grasped using conventional fishing tools. This is especially true for small metal objects and, in particular, small metal objects that have an irregular shape or small pieces that can be broken up during the retrieval process. Unfortunately, drilling rigs typically have many small metal objects (such as, for example, wrenches, chain, bolts, tong dies and nuts) at or near the rig floor. Such objects, which are in relatively close proximity to the upper opening of a well, are at risk of falling into a wellbore.
As noted above, such relatively small metal objects can cause significant disruptions to downhole operations. Further, fishing operations for small metal objects can be very time consuming and, as a result, very costly. Accordingly, the best way to prevent such disruptions and to avoid long and expensive fishing operations for such small objects is to keep such objects from entering a wellbore in the first place.
Rig operating procedures frequently dictate that when no pipe is present in a wellbore that blind rams in a blowout preventer (“BOP”) assembly be closed in order to block access to subterranean portion of said wellbore and keep any unwanted falling objects from entering said wellbore from above the BOP assembly. If an object is dropped into a well at the rig floor, with such BOP blind rams closed, the object will not fall all the way into the subterranean portion of a wellbore.
However, this solution is less than optimal, because the object must still be retrieved from the top of the blind rams before operations can resume. Such retrieval process typically requires draining such BOP assembly of fluids to locate the object, opening the bonnet in the BOP assembly, finding and retrieving the object, and then closing and retesting the BOP assembly to the required test pressures. This retrieval process—while frequently quicker and less expensive than fishing the item from the bottom of the well—is nonetheless still relatively expensive, time consuming and dangerous for personnel.
Thus, there is a need for an apparatus and method for catching dropped objects, and particularly metallic objects, before such objects enter the subterranean portion of a well. Such apparatus and method should prevent dropped objects from falling further into a wellbore, and should hold such objects for ultimate retrieval and removal from a wellbore. As an added benefit, such apparatus should also allow for the removal of metallic debris from well fluids.